1. Field of the Invention
The present invention relates to the stimulating of subterranean formations surrounding oil wells, gas wells, water wells, injection wells and similar boreholes.
2. Brief Description of the Prior Art
The flow of oil from a subterranean formation to a well bore depends, among other factors, upon the permeability of the formation. Often permeability is not sufficient to allow a desired flow rate of fluids, such as, crude oil, natural gas and so forth from the formation. In such a case, the formation can be treated to increase its permeability.
Acid treating has been widely employed for carrying out the treatments in subterranean oil, gas, or water producing formations. Such acidizing treatments have been used primarily to increase the productivity of oil and gas from a calcareous formation by bringing about the removal of reactive materials from naturally occurring fractures and pore spaces in the formation whereby the sizes thereof are increased. The usual technique of acidizing the formation comprises introducing a non-oxidizing acid into the formation under sufficient pressure to force the acid into the formation where it reacts with acid soluble components of the formation and thereby enlarges the interstices of the formation with an attendant increase in permeability.
Acidizing fluids can also be utilized during the stimulation of a well to remove acid soluble scales present in the well bore or to dissolve fine particles which have been formed in the perforating process and are present in the vicinity of the well bore. Generally, this technique involves placing a small quantity of acid in a desired portion of the well bore and allowing the acid to react with the scale deposits that are desired to be removed. Alternatively, the acid may be circulated across the perforations or formation face to dissolve the unwanted deposits.
Hydraulic fracturing also can be employed to increase the production of fluids from a subterranean formation; and accordingly many methods and compositions useful for hydraulically fracturing a subterranean well formation penetrated by a well bore have been developed. Commonly, in the art of hydraulic fracturing, a fluid is introduced into the formation sought to be fractured by a conduit, such as tubing or casing, disposed in a well bore. The fluid is introduced at a rate and pressure sufficient to produce a fracture in the formation and to extend the produced fracture from the well bore into the formation. The fluid can include a propping agent, which results in placement of the propping agent within the fracture thus produced. Following the fracturing treatment, the introduced fluid is recovered from the formation, but the proppant remains in the produced fracture to thereby prevent the complete closure thereof and to form a conductive channel extending from the well bore into the formation. Combination fracture acidizing processes are well known in the art.
The presence of iron in subterranean formations presents a troublesome problem during the stimulation of the formations.
Whenever a formation containing iron salts or oxides is acidized, acid soluble iron compounds are dissolved at first by the acid; but such compounds remain dissolved in the acidic solution only as long as the pH is less than that at which they are normally precipitated. As the acid solution continues to act upon the formation deposits and becomes spent, the pH of the solution increases to about 4 to 5. Ferric iron compounds begin to precipitate at a pH of about 2.5 and are precipitated when the pH is about 3.5. Thus, the acid solution when spent, no longer retains the ferric iron in the solution and the resulting iron precipitates often reduce permeability by objectionably plugging, clogging or otherwise obstructing the pore spaces and other openings providing fluid passageways in the well stratum.
A problem encountered in the fracturing of a subterranean formation is that the water which is present in the formation can contain dissolved ferrous ions. If the fracturing fluid that is injected into the formation contains oxygen, the oxygen will oxidize the dissolved ferrous ions to ferric ions and this results in precipitation of the ferric ions when the pH of the formation water is greater than 2.5. This, also causes objectionable plugging, clogging or obstruction of the pore spaces and other openings in the fluid passageways of the subterranean formation.
Still another problem encountered in stimulating a subterranean formation is the presence of iron in the aqueous fluids that are used for stimulating the formation. These aqueous fluids are frequently mechanically blended in order to insure proper mixing of the components in the fluid and are pumped into the subterranean formation. During the blending and pumping of the stimulating fluids, any ferrous ions present in these stimulating fluids may be oxidized to ferric ions. Since these ions begin to precipitate at a pH of about 2.5, and are precipitated when the pH is about 3.5, the stimulating fluid can deposit the ferric ions on the pore surfaces of the subterranean formation and thus cause low permeability of the formation.
In order to prevent the precipitation of the iron compounds from spent acidizing solutions, sequestering additives have heretofore been utilized in acidizing solutions. For example, acetic acid has been used as a pH control agent for keeping iron and the like in solution in spent acid. U.S. Pat. No. 3,142,335, which is assigned to the assignee of the present invention discloses an iron sequestering additive for use with hydrochloric acid solutions comprised of a mixture of citric acid or a salt thereof, and a low molecular weight organic acid or salt thereof, such as acetic acid or formic acid.
Numerous other patents disclose the use of chelating agents for preventing the deleterious effects of dissolved iron in well treatments involving aqueous strong acids. U.S. Pat. No. 2,715,081 discloses an acidizing composition made up of a strong mineral acid containing sulfurous acid, which prevents the precipitation of iron in the formation. U.S. Pat. No. 2,175,079 suggests that the inclusion within an acidizing fluid of a material such as lactic acid, ammonium acetate, glycine, glycolic acid, citric acid or the like keeps the iron compounds dissolved by the acid in the solution. U.S. Pat. No. 3,150,081 discloses the use of hydroxyacetic acid and citric acid to prevent the precipitation of iron during acidizing operations.
U.S. Pat. No. 4,574,050 discloses the use of ascorbic acid, erythorbic acid, their salts or .gamma.-lactone of such acids to prevent the precipitation of iron during acidizing operations. It would be desirable to provide a composition and method for preventing the precipitation of iron during the stimulation of subterranean formations.